I had the same issue the first time I used the app, because like you I was looking for that option in the settings page, where it is in 1P7.
In 1P8 for iPhone:
- Go to the main screen (“Home” icon, bottom left);
- Tap on your Display Picture (top left);
- Tap on “Set Up Another Device…”;
- Tap on “Setup Code” and the QR will show up.
On iPad the only difference is that there is no “Home” screen and the Display Picture is on the sidebar which is always active in landscape mode but disappears in portrait mode.
I hope this helps.
1Passerby